if the operation is what i'm picturing, you could mark the line to which the flats should be extended on the shaft (using bluing or just scribe it on the metal), then use a file and work towards it. but smooth (i.e. ruin) the edge of the file that runs along the flat so you don't accidentally cut into that surface.
if the operation is what i'm picturing, you could mark the line to which the flats should be extended on the shaft (using bluing or just scribe it on the metal), then use a file and work towards it. but smooth (i.e. ruin) the edge of the file that runs along the flat so you don't accidentally cut into that surface.
does that make sense?